Young's double slit gone wrong

In summary, the experiment worked better when the laser beam was centered on the slits and the gap between them was smaller.
  • #1
Hi Everyone

I was trying to do a young's double slit experiment with green laser.

The gap between the slits was d = 0.001m

The distance to the screen was D =1.2m

We counted from the center 10 bright fringes m = 10

The distance of the from the center bright fringe was y = 0.067 m

Using a small angle approximation: y = (mλD)/d

So λ = (yd)/mD = (0.067 x 0.001)/12 = 5.5 x 10^-6 Wavelength of laser given as about 550 nm = 5.7 x 10^-7

ouch! out by a factor of ten. Can't figure out what I did wrong.

NB: This is not a homework question. I am just trying to make the experiment work, and though I am ashamed of my incompetence, I am actually the teacher!

Thanks in advance for any suggestions. The bright spot in the middle was a bit big and smudged. But that was partly why we chose to count as many as 10 fringes. Even if for some reason there were an extra one or two fringes that got burnt out at the center, that would not account the factor of ten.
 
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  • #2
Was this done on an optical table? Do you know the coherence length of your laser?

You will get the best results with a HeNe on a table which cannot vibrate ... the basement floor is better than a lab bench on the second story!

From your use of a green laser I suspect you are using a laser pointer ... in that case you may also have a problem with the fixturing.

I did not check the math ... I was immediately drawn to the smudged central spot.
 
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  • #3
Thanks. The thing was/is that the pattern of bright and dark fringes was very clear. The set up was crude. But I don't understand why I should be so neatly out by a factor of ten. I think I would like to try some more simple single slit diffraction experiments and set what happens there. I wasn't really looking for ultra precision, just a quick demonstration to show the students and hopefully to get some practice with the calculations. In fact I am beginning to wonder whether what I was actually looking at was the diffraction pattern cause by only one slit. The slits themselves were about 1/10 th of a millimeter. Thanks, I will go back and try again.
 
  • #5
Yes, I suspect you're counting the single-slit maxima, not the double-slit maxima. If the width of the slits is about 1/10 the spacing between them, that would account for the numbers you're getting. Try increasing D to a few meters and doing the experiment in a dark room. That spreads out the pattern and you can distinguish more easily between the single-slit and double-slit components of the interference pattern:

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/dslit.html
 
  • #6
jtbell said:
Yes, I suspect you're counting the single-slit maxima, not the double-slit maxima. If the width of the slits is about 1/10 the spacing between them, that would account for the numbers you're getting. Try increasing D to a few meters and doing the experiment in a dark room. That spreads out the pattern and you can distinguish more easily between the single-slit and double-slit components of the interference pattern:

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/dslit.html

That seems likely to me, too. The width / spacing ratio was 1:10. Coherence shouldn't be a prob as you can do Young's Slits with a discharge lamp and a single collimating slit. Just increase the throw and the finer fringes should jump out at you.
 
  • #7
If this is a single-slit diffraction pattern, then the spacing between the two dark fringes nearest the center will be twice the spacing of other adjacent fringes. For a double slit, the spacings would be equal.
 
  • #8
Done in open air, decoherence?
 
  • #9
I did the experiment again. This time I drilled a single hole through a piece of metal. The whole was 0.3 mm and the resulting interference pattern was easy to see. The calculation produced a result a result that was about 95% accurate. This was done with a green laser pointer clamped in a regular lab clamp stand, and projected over a distance of just over a meter. So problem solved. Thanks everyone for your replies and advise
 
  • #10
That is one small drill bit. Just FYI, the pattern from a single hole is considered a diffraction pattern, not an interference pattern.

Glad it worked out.
 
  • #11
The light doesn't care what you call it. Is there any practical example of 'pure' interference?
People are obsessed with classification.
 

What is Young's double slit experiment?

Young's double slit experiment is a classic experiment in physics that demonstrates the wave nature of light. It involves shining a beam of light through two narrow slits and observing the interference pattern created on a screen behind the slits.

What can go wrong in Young's double slit experiment?

There are a few common issues that can occur in Young's double slit experiment. These include: 1) Light from other sources interfering with the experiment, 2) The slits being too wide, causing diffraction instead of interference, and 3) The screen being placed too close to the slits, resulting in a blurry interference pattern.

How can we fix these problems?

To prevent outside light from interfering, the experiment should be conducted in a dark room or with a light-blocking enclosure. The width of the slits can be adjusted by using a narrower source of light or by using a different material for the barrier with the slits. The screen should be placed at a distance that allows for a clear interference pattern to form.

Why is Young's double slit experiment important?

Young's double slit experiment is important because it provides evidence for the wave theory of light and helps explain the phenomenon of interference in waves. It also paved the way for further studies and developments in the field of quantum mechanics.

Can Young's double slit experiment be used to study other types of waves?

Yes, the principles of Young's double slit experiment can be applied to other types of waves, such as sound waves and water waves. This experiment is a fundamental tool for understanding the wave nature of various phenomena in the natural world.

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